Mullerian Inhibiting Substance is a macromolecular glycoprotein which causes regression of the Mullerian ducts in the mammalian male embryo. Mullerian Inhibiting Substance is present in the postnatal testes of the rat, human, and calf. The postnatal calf testes has been the source for biochemical isolation in this laboratory. A specific constituent extracted from bovine testes has been characterized by density gradient sedimentation, gel chromatography, affinity chromatography, ion exchange chromatography, gel electrophoresis, isoelectric focusing, amino acid analysis, and gas liquid chromatography. Mullerian Inhibiting Substance activity of testicular fragments and of each biological fraction have been verified in an organ culture assay. Antibody to the testicular extract and active fractions has been raised in rabbits and absorbed with fetal calf serum. Using gel electrophoresis and isoelectric focusing, single bands will be tested in a modified organ culture system for Mullerian Inhibiting Substance activity, and active bands will be used to raise a more specific antibody, in the hope of developing a radioimmunoassay. Ultrastructural studies have demonstrated a sequence of lysosome stimulation, autodigestion, and autophagocytosis, leading to duct regression both in vivo and vitro. Mesenchymal-epithelial, basement membrane, and extracellular matrix interactions will be studied during duct regression. The Sertoli cell will be studied during maximum Mullerian Inhibiting Substance production. Gonadotropins inhibit testicular release or production of Mullerian Inhibiting Substance. The specific role of FSH as an extrinsic modulator has been confirmed. Microcytotoxicity techniques have been used to demonstrate the sensitivity of Mullerian duct cells and tumors of Mullerian duct origin to Mullerian Inhibiting Substance. Mullerian Inhibiting Substance will be explored as a potential chemotherapeutic agent against tumors of Mullerian duct origin.